We will study the role of macrophages, which appear to act as primary effector cells in conferring protection from lethal viral-induced central nervous system disease. Preliminary studies show that SJL mice exhibit an age-dependent resistance to central nervous system infection due to the neurotropic JHM strain of mouse hepatitis virus. Resistance can be transferred from old (12 week) resistant mice to young (6 weeks) susceptible animals with either splenic or peritoneal macrophages taken from animals which exhibit no evidence of immunity to mouse hepatitis virus. Macrophages from 6 and 12 week old animals will be compared to determine if additional age-dependent changes in macrophage functions can be found. The peritoneal exudate cells from the two age groups will be compared for the number of total macrophages; the level of "activation"; the numbers of Ia antigen, Fc, and C3 receptor bearing cells; and the ability to function as phagocytes, as ADCC effectors, and as T and B cell helpers.